Anaerobic composite matrix resins

ABSTRACT

A matrix resin composition for fiber reinforced composite materials is described. The resin is thermosetting and achieves a glass transition temperature of at least 177° C. (Tg), obtained by curing at room temperature. The matrix resin will streamline composite fabrication processes by eliminating the need for heating during the cure process. The implications of this development are significant in terms of the ease of use and elimination of procedural steps. While the resin system was developed specifically for vacuum bagging, it is expected to be viable for other composite fabrication methods including resin transfer molding (RTM) and vacuum-assisted resin transfer molding (VARTM). The resin system is viable for use with carbon fiber reinforcements to fabricate laminates at least 0.20 inches thick. The resulting laminates have low porosity and mechanical properties equivalent to those prepared with common epoxy matrix resins.

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/571,349, filed Oct. 12, 2017, entitled “Anaerobic CompositeMatrix Resins,” the entire contents of which are hereby incorporated byreference.

This invention was made with government support under Contract No.N68335-16-C-0242 awarded by the U.S. Navy, Naval Air Systems Command Thegovernment has certain rights in the invention.

The present disclosure pertains to novel composite materials useful forcomposite repair and composite part fabrication.

BACKGROUND

Materials and techniques for fabrication of fiber reinforced compositematerials that exhibit elevated temperature performance have severallimitations. A common measure of thermal performance is the deliveredglass transition temperature (Tg) of the fabricated composite. Matrixresins for elevated temperature composite applications typically havetwo-components that require mixing, or are pre-impregnated on the fiberreinforcement. The pre-impregnated versions suffer from short shelf-lifeand must be stored at sub-ambient temperatures to prevent prematuregellation of the resin system. Elevated Tg matrix resins commonly haveexcessive viscosities, which require heating during processing to reduceviscosity and maximize consolidation with the fiber reinforcement.

High temperature laminating resins often require elevated temperaturecuring. Resins with elevated Tg that cure at ambient temperatures havepreviously been developed that cure via ultraviolet light (UV). However,these UV curing acrylate resins are limited to use with fiberglass orquartz fabrics. Their efficacy with carbon fiber reinforcements isproblematic due to the strong UV absorption of the carbon.

What is needed is a composite fabrication system that does not requireheat or UV energy to initiate cure, yet that delivers composites capableof high temperature service conditions.

SUMMARY

The present disclosure pertains to low viscosity ambient temperaturecuring composite matrix resin systems that eliminates the need forheating to achieve cure. Compared to previous resin technologies, thecomposite resin system will cure completely when carbon fiber is used asthe reinforcement. The resin is also compatible with fiberglass andquartz reinforcements.

The resins have the potential to reduce costs currently associated withcomposite fabrication. The resin system can be used for originalcomposite part fabrication or for repair of damaged composite parts.With regards to the latter, the matrix resins developed will providerepairs having equivalent strength, while reducing the support equipmentand man-hours per repair. The resin system developed can be cured atambient temperatures. Without postcure the resin will provide a glasstransition temperature more than 350° F. (177° C.). The resultingcomposites also exhibit high fiber strength translation. Theimplications are significant in terms of the ease of use and eliminationof procedural steps. While the resin system was developed specificallyfor vacuum bagging, it is expected to be viable for other compositefabrication methods including out of autoclave (OOA), resin transfermolding (RTM) and vacuum-assisted resin transfer molding (VARTM).

Acrylate based resin systems were formulated to cure anaerobically. Theresult is a resin that is stable at room temperature for months in thepresence of atmospheric oxygen. Therefore, one embodiment is a stableprepreg material (i.e., fabric reinforcement that has been“pre-impregnated” with a resin system) that will only cure afterapplication to the repair area when under vacuum bagging conditions,which removes oxygen and allows cure of the part at ambienttemperatures.

Resins that cure anaerobically were first discovered in the 1940′s, whenit was found that acrylate based adhesives formulated with specificcuratives, form metal-to-metal bonds in the absence of oxygen. A keyfactor in the cure mechanism is the need for metal catalysis. For metalbonding applications, the catalytic metal is supplied at the substrateinterface. Copper and iron are well known to increase kinetics ofreaction whereas cadmium or zinc are inactive. After the adhesive isapplied, which removes access to oxygen, peroxides form free radicalsunder the catalytic effect of metal ions.

Anaerobically curing matrix resins for composite applications has notbeen investigated. The reason for this lack of research in the use ofanaerobic resins for composites is likely due to the absence of metalions on the surface of common reinforcements.

The matrix resin technology described herein is based on an anaerobiccuring approach that will potentially transform not only compositerepairs, but several composite fabrication areas in general. Embodimentsof the technology have been demonstrated whereby a unique sizing agentsystem is used to treat reinforcements with organometallic compounds,which serve as catalysts in the anaerobic reaction. When the formulatedmatrix resin is applied to the treated reinforcement, cure is notinitiated until the impregnated fabric is exposed to an oxygen freeatmosphere. This occurs during the vacuum bagging process, where theresin cures to a rigid cross-linked network at ambient temperatures. Thecuring agent package, which composes less than 5% of the formulation inpreferred embodiments, is based on the proper balance of aromaticamine(s) and hydroperoxides, and saccharin. The curing agent package mayinclude combinations of peroxide initiators, such as cumenehydroperoxide, aromatic amine accelerators, and benzoic sulfimide(saccharin). Additional preferred embodiments also use a curepromoter/silane adhesion promoter applied to the carbon fiber fabricreinforcement.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the anaerobic cure profile for a preferred embodiment of aresin system with carbon fiber reinforcement and promoter solution at23° C.

FIG. 2 shows the effect of promoter concentration applied to fiber oncure time at 23° C. with a preferred embodiment of a resin system.

FIG. 3 shows a 20 ply AS4/plain weave carbon fiber laminate producedusing a preferred embodiment of an anaerobic epoxy acrylate matrixresin.

FIG. 4 shows a cross section of cured plain weave carbon fiber laminateproduced using a preferred embodiment of an anaerobic epoxy acrylatematrix resin.

FIG. 5 shows a simulated repair performed on 2.0 in. Honeycomb sandwichcomposite.

FIG. 6 shows a complete repair panel fabricated with treated plain weavecarbon fiber and a preferred embodiment of a matrix resin.

FIG. 7 shows the short beam shear results for IM7 unidirectionalcomposite prepared with a preferred embodiment of an anaerobic resin.

FIG. 8 shows the glass transition temperature for a preferred embodimentof a resin system reinforced with carbon fiber.

FIG. 9 shows the glass transition temperature for a commerciallyavailable resin reinforced with carbon fiber after cure.

FIG. 10 shows the thermal stability of a preferred embodiment of a resinand a standard epoxy resin composite with plain weave carbon fiberfabric.

FIG. 11 shows photomicrographs of a composite cross section using apreferred embodiment of a resin.

FIG. 12 shows photomicrographs of a composite cross section using acommercially available resin.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present disclosure relates to anaerobically curing composite resinsystems. Preferably, the system is composed of acrylate based resinmaterials and curing agents that promote cure under anaerobicconditions. An additional preferable aspect involves treatment of thefiber reinforcement with an organometallic catalyst sizing system, alsoreferred to as a promoter/silane adhesion promoter solution, oractivator sizing agent. The treated reinforcement is therefore rendereda part of the curing mechanism. The composite system is designed to cureonly when the resin comes into contact with the fiber reinforcement andis in an anaerobic state. Such anaerobic conditions commonly occur incomposite fabrication methods such as vacuum bag molding and closedmolding.

Anaerobically curing resins commonly contain a free-radicallypolymerizable acrylate ester monomer, together with a peroxy initiatorand an inhibitor component. Often, such anaerobic resins also containaccelerator components to increase the cure speed of the composition.

The basic components in preferred embodiments of the matrix resininclude acrylate based resin materials, including acrylate monomers andpolymers blended to provide the desired mechanical and thermalproperties. Useful acrylates include monomers and oligomers derived frombisphenol-A dimethacrylate, hydrogenated bisphenol-A dimethacrylate, andethoxylated bisphenol-A dimethacrylate. Additional preferred monomersinclude methyl methacrylate, methacrylic acid, tris (2-hydroxy ethyl)isocyanurate triacrylate, Isobornyl methacrylate, tetrahydrofurfurylmethacrylate, and tricyclodecane dimethanol diacrylate, andhexafunctional aromatic urethane acrylate. Various usefulurethane-acrylate type monomers include those derived from chemicallinking of precursor “prepolymers” then “capping” with (meth)acrylate.

A description of examples of preferred acrylate monomers and oligomersused for formulation of preferred embodiments of the matrix resins isshown in Table 1 below.

TABLE 1 ACRYLATE MONOMERS AND OLIGOMERS Description/ [Commercial ProductResin Component Number] Structure Epoxy acrylate oligomer Impartsflexibility, excellent adhesion, and low shrinkage [CN UVE 151, Sartomer(Exton, PA)]

Tris (2-hydroxy ethyl) isocyanurate triacrylate Fast cure response,adhesion, weatherability, high impact strength, low shrinkage andhardness. [SR368, Sartomerl

Isobornyl methacrylate Excellent reactive diluent for oligomers. Thecyclic group produces polymers through Tree radical curing which have ahigh glass transition temperature. [SR423A, Sartomer]

Hexafunctional aromatic urethane acrylale oligomer A hexafunctionalaromatic urethane acrylate oligomer with excellent cure response, lowviscosity, and very high crosslink density. [CN975, Sartomer]

Tetrahydrofurfuryl methacrylate (THFMA) Monofunctional methacrylatemonomer offers low viscosity, low shrinkage, high hardness, highsolvency, good adhesion, good balance of mechanical properties

Tricyclodecane Dimethanol Diacrylate Low viscosity difunctional acrylatemonomer that provides high Tg, high flexibility, and low shrinkage.

The curing agent package may include combinations of peroxideinitiators, such as cumene hydroperoxide, aromatic amine accelerators,and benzoic sulfimide (saccharin). Common initiators include one or moreof cumene hydroperoxide, t-butylhydroperoxide, p-menthane hydroperoxide,diisopropylbenzene hydroperoxide, pinene hydroperoxide, and methyl ethylketone peroxide. Anaerobic cure-inducing compounds to accelerate curecan include saccharin and an aromatic amine Examples of preferablearomatic amines include one or more of N,N-diethyl-p-toluidine,N,N-dimethyl-o-toluidine, and acetyl phenylhydrazine (APH),N,N-dimethylaniline, N,N-dimethyl-p-toluidine, N,N-dimethyl-p-anisidine,N,N-diethylaniline, and N,N-bis-(2-hydroxyethyl)-p-toluidine.

Various additives can be added to the resin formulation, and stabilizersare typically added to prevent premature polymerization. The addition ofstabilizers is important to maximize long-term room temperaturestability. Preferred stabilizers include chelators such as tetrasodiumethylenediamine tetraacetic acid to scavenge extraneous metal ions.Radical inhibitor additives may also be included in the formulation,such as hydroquinone or naphthoquinone. Additives for viscosity controlinclude fumed silica, also known as pyrogenic silica.

The composite materials can be produced by conventional manufacturingprocesses that are capable of anaerobic conditions. These processesinclude Resin Transfer Molding (RTM), Vacuum Assisted Resin TransferMolding (VARTM), vacuum bag molding, and filament winding. However,compared to previous state of the art materials, these new materials donot require elevated temperature cure to achieve the desired high glasstransition temperatures. Thus, the cost associated with heatingequipment and energy usage is expected to be lower compared toconventional composite materials.

One preferred embodiment is the use of the anaerobic resin system withcarbon fiber reinforcements that have been treated with compounds tofacilitate cure and adhesion of the matrix resin. A solution containingaccelerators and adhesion promoters can be used as a fiber sizing agentfor the carbon fiber. The accelerator/adhesion promoter solution isapplied to the carbon fiber and allowed to dry, leaving residualaccelerator/adhesion promoter coating in the form of organometalliccompounds on the reinforcement material. After evaporation, the amountof accelerator/adhesion promoter composition that adheres to the carbonfiber is preferably in the range of 0.1 to 5 percent based on the weightof the carbon fiber.

Particularly suitable accelerators include compounds containingtransition metal ions. Preferred transition metals include copper,manganese, chromium, iron, cobalt, nickel, and molybdenum. Morepreferred is copper. The oxidation state of the transition metal is notcrucial, but the lower oxidation state which can be oxidized is ratherpreferred. The transition metal compound may be in the form of aninorganic or organometallic compound, including oxides, salts, andorganometallic chelates and complexes. Appropriate inorganic saltsinclude the sulfates, nitrates, chlorides, bromides, phosphates, andsulfides. Suitable organic salts include the alkoxides, includingmethoxides and ethoxides, as well as the carboxylates, including theacetates, hexoates, octoates, ethylhexanoates, and naphthenates. Othersuitable transition metal complexes include the acetylacetonates and thehexafluoroacetylacetonates. More preferably, the transition metalcompound is selected from the group consisting of copperacetylacetonate, copper 2-ethylhexanoate, copper acetate, coppernaphthenate, copper octoate, copper hexoate, and copperhexafluoroacetylacetonate. Most preferably, the transition metalcompound is copper acetylacetonate.

Preferred adhesion promoters include amino silanes, such asgamma-aminopropyltrimethoxy silane, gamma-aminopropyltriethoxy silane,N-(betaaminoethyl)-gamma-aminopropyltriethoxy silane, and the like.However, other organo silanes can be utilized as well as thecorresponding silanols and polysiloxanes.

An example of accelerator/adhesion promoter adhesion promoter solution,or activator sizing agent, includes copper acetylacetonate (2% w/w) andgamma-aminopropyltrimethoxy silane (1%) in methylene chloride. Lowerconcentrations of both the accelerator and adhesion promoter were foundto be effective. The preferred range for the accelerator component is0.2 to 5% weight in solution. The preferred range for the adhesionpromoter is 0.1 to 3 percent by weight in solution.

Preferred embodiments described herein are preferably used as a matrixresin for carbon fiber reinforced composites. The types of carbon fiberthat can be used with this resin include unidirectional and wovenproducts. These carbon fiber reinforcements are available from severalmanufacturers including Toray Industries Inc., Toho Tenax Co. Ltd.,U.S., Zoltek Companies Inc., and Hexcel Corp.

The preferred curing method involves inducing an anaerobic state aroundthe impregnated carbon fiber reinforcement. Some composite fabricationtechniques employ vacuum assistance as part of the impregnation andcompaction process. The use of vacuum inherently produces an anaerobicstate, wherein the materials of the present invention would be viable.Examples of this include vacuum bag molding and resin transfer molding.Other composite fabrication techniques could be modified to introduce ananaerobic state, either by use of vacuum or purging the processequipment or molds with a gas that is oxygen-free.

EXAMPLE 1. EXEMPLARY FORMULATIONS

Carbon fiber reinforcement cloth used in the following examples wasHexcel style 282 made with AS4 input fiber.

Combinations of the acrylate monomers and oligomers listed in Table 1can be used in preferred embodiments of the matrix resin. Oneparticularly advantageous preferred embodiment of a resin formulation isdescribed in Table 2 below. This Example resin system was designatedinternally as AC-1291. It contains methyl methacrylate, methacrylicacid, epoxy acrylate, tris (2-hydroxyethyl) isocyanurate triacrylate,hexafunctional aromatic urethane acrylate, and isobornyl methacrylate.The curing agent components included saccharin, N,N-dimethylaniline, andcumene hydroperoxide.

TABLE 2 AC-1291 ACRYLATE RESIN SYSTEM Component % Resin components:Methyl Methacrylate 7.89 Methacrylic Acid 9.86 Epoxy acrylate 42.31 Tris(2-hydroxyethyl) isocyanurate 20.19 triacrylate Isobornyl methacrylate12.62 Hexafunctional aromatic urethane acrylate 4.89 Curativecomponents: Saccharin 0.79 N,N-Dimethylaniline 0.14 Cumene Hydroperoxide1.31 Total 100.0

In an example, HEXCEL (Stamford, CT) HEXTOW IM7 carbon fiber was treatedwith a solution of gamma aminopropyltrimethoxy silane (1% w/w) andcopper acetylacetonate (2% w/w) dissolved in methylene chloride. Thesolvent was allowed to evaporate from the carbon fiber, thus leaving anactivated sizing component on the reinforcement. Eighteen strands of thetreated carbon fiber reinforcement were then pulled into a thin flexibletube. The tube was then injected with the anaerobic epoxy acrylate resin(AC-1291), while fixtured in a dynamic mechanical analyzer (DMA). Thedynamic modulus of the composite was measured at room temperature untilno additional increase in modulus was observed. FIG. 1 shows the curestudy results, the anaerobic cure profile for AC-1291 resin system withIM7 carbon fiber (2% promoter on fiber) at 73° F.

The cure rate was found to be very rapid when the copper acetylacetonateaccelerator concentration in the fiber treatment was 2 percent, showinga cure onset of approximately 3 minutes. The specimen achieved full cureat room temperature within one hour of combining the anaerobic resin andactivating carbon fiber fabric. After ambient temperature cure the glasstransition temperature for this composite was determined to be 360° F.(182° C.) using DMA in the flexural mode.

Because of the rapid cure observed at room temperature with the twopercent accelerator treatment, additional tests were performed using arange of promoter concentrations (0.008% to 0.25%.) FIG. 2 shows theeffect of varying promoter level (applied to fiber) on level of cure at73° F. for AC-1291, which is expressed as conversion relative to theultimate modulus on the plot.

EXAMPLE 3. COMPOSITE FABRICATION

Woven carbon fiber fabric (HEXCEL HEXTOW AS4) was pretreated with asizing containing 0.5 percent copper acetylacetonate accelerator and 1.0percent gamma aminopropyltrimethoxy silane (adhesion promoter) dissolvedin methylene chloride. After evaporation of the solvent, five plies ofthe woven carbon fiber fabric were impregnated with resin AC-1291 anddebulked/cured in a vacuum bag for thirty minutes. The laminate whichwas approximately 4 inches square was allowed to cure for thirty minutesunder vacuum. The exotherm was measured with a thermocouple and themaximum temperature recorded was 65° C. Larger panels were subsequentlyprepared using this same process.

Fabrication of larger test laminates was performed for evaluation ofcomposite mechanical and thermal properties. The resin used for thesetrials was the preferred anaerobic resin AC-1291. No heating blanketswere required and the laminate was simply cured by removal of oxygenduring the vacuum bagging procedure. The AS4 carbon fiber was treatedwith the activator sizing agent (0.2% copper acetylacetonate(accelerator) and 1.0% gamma aminopropyltrimethoxy silane dissolved inmethylene chloride. In this case the CuAcAC concentration was reduced to0.2% in order to provide additional working time and to reduce excessexotherm.

The resin maintained a low viscosity during the lay-up process. Afterpulling a vacuum on the part (25 in. Hg.) The temperature of thelaminate began to increase as the anaerobic reaction was initiated. Themaximum temperature measured from the exotherm was approximately 65° C.The ability to fabricate laminates up to 0.20 inches thick wasdemonstrated. The photograph in FIG. 3 shows the composite laminateprepared with the developed resin (AC-1291) and 20 plies of AS4 plainweave carbon fiber fabric. FIG. 4 is a photograph of the cured laminatecross section.

Composite sandwich panels are composed of thin, high strength compositeskins separated by and bonded to lightweight honeycomb cores. Thesestructures are commonly repaired on aircraft and require additionalsteps to ensure that mechanical properties are restored to the damagedarea.

A simulated 0.5 inch thick sandwich panel repair was prepared by cuttinga 2.5 in diameter circular area from one of the laminate sides. Five AS4carbon fiber patches, previously treated with activator, were cut tofill the void, and one final patch was applied that was approximatelytwo inches larger in diameter. Each fabric patch was wet-out withAC-1291 resin. A second simulated repair was performed on a 2.0 inchthick sandwich panel using the same basic materials and process. Aphotograph of this cured repair after debagging is shown in FIG. 5.

Laminate repair simulations were performed using previously preparedepoxy/carbon fiber flat panels, 3 ft.×1 ft.×0.168 in. An 8-inch diametercircular area was abraded in the middle of each panel, using a 90 degreedie grinder and Scotchbrite pads. Plain weave AS4 carbon fiber cloth,that was previously treated with activator (0.2%), then cut intocircular repair plies.

The circular plies were used to lay-up two simulated repairs, using a“wedding cake” stack configuration to simulate surface doubler repairs.For actual repairs, a tapered scarf repair would typically be done, withthe repair plies laid with the smallest ply down first, then the nextsmallest ply, and so on, with the largest ply being the top repair ply.The reason that the lay-up was prepared in reverse order, with the largeply first and the smallest ply on the top, was to make each repair plyvisible during fatigue testing of the cured repair section.

The previously abraded composite area was wet out with the AC-1291resin, followed by laying the 7-inch diameter ply into the wet resin andadditional resin on top of the ply, using the stiff short-bristle brushto work the resin down through the thickness of the ply. Afterthoroughly wetting out the first ply, additional resin was applied towet out the subsequent ply. This process was repeated for all succeedingplies, with the 1-inch circular ply being applied last. Care was takento ensure that all plies were thoroughly saturated with resin. All plieswere laid up as symmetrical 0°/90° plain weave plies, to match theoriginal structure.

The two sets of repair plies were wet out in the same way, from the samebatch of mixed resin. Up to this point, the two repairs were treatedidentically. The difference between the two repair panels involved thevacuum bag bleeder schedules used. Panel 1 was bagged with an aggressivebleeder schedule, designed to pull out excess resin under vacuum.

Both parts (Panels 1 and 2) were connected to a vacuum pump at separatetimes. Panel 1 was connected to vacuum approximately 30 minutes prior tovacuum being applied to Panel 2. The vacuum pressure was maintained at24 in Hg to extract air from both bags. Panel 1 was under full vacuumfor 3 hours, and Panel 2 was under full vacuum for 2.5 hours. No thermalcure was required. When the panels were debagged, the resin appeared tobe fully cured, and was hard to the touch, with no trace of tackiness.

After debagging both panels, it was noted that Panel 2 had a more resinrich surface compared to Panel 1. This was attributed to the moreconservative bleeder schedule used for Panel 2. Photographs of the curedrepair laminates are shown in FIG. 6.

EXAMPLE 4: TESTING OF COMPOSITE LAMINATES

Interlaminar shear strength (ILSS) tests were performed according toASTM Test Method D2344, Standard Test Method for Short-Beam Strength ofPolymer Matrix Composite Materials and Their Laminates(astm.org/Standards/D2344.htm). Specimens were prepared using theAC-1291 resin system with IM7 carbon fiber reinforcement that waspreviously treated with a methylene chloride solution containing 0.2percent copper acetylacetonate and 1 percent gammaaminopropyltrimethoxysilane. The carbon fiber was wet-out using AC-1291anaerobic resin.

Forty two ends of the IM7 carbon tow wet-out with AC-1291 resin. Thewetted fiber bundle was the pultruded through an enclosed metal dietubing section which removed oxygen availability from the resin. Thisinduced the anaerobic cure conditions. The specimens' were allowed tocure at ambient temperature for 24 hours. After cure, the unidirectionallaminate was extracted from the metal tubing and was cut into smallerspecimens which were 1.0 in×0.17 in×0.17 in.

The individual ILSS specimens were positioned in an Instron test machinein a three point bend configuration. Support span was 0.63 inches, andthe crosshead speed was 0.05 inches per minute. The maximum load wasused to calculate the interlaminar shear strength.

FIG. 7 shows short beam shear results for IM7 unidirectional compositeprepared with anaerobic resin AC-1291. The data presented in FIG. 7shows that the AC-1291 matrix resin can deliver strengths comparable tolaminates prepared with the standard EA-9390 matrix resin. An averageILSS of 8,570 psi was obtained.

Flexural properties were determined for the anaerobically curedcomposite prepared with 20 plies of AS4 carbon fiber fabric. The fabricwas treated with a methylene chloride solution containing 0.2 percentcopper acetylacetonate and 1 percent gamma aminopropyltrimethoxysilane.The carbon fiber was wet-out using AC-1291 anaerobic resin. A vacuum bagassembly was used to remove oxygen from the bagged laminate to promotecure. Vacuum was maintained for one hour at ambient temperature.

Four-point bend flexural tests were performed using composite specimenscut from test panel. Specimens having dimensions of 8 in.×0.5 in×0.20 inwere used. The support span was six inches and the load span was 3inches. A crosshead speed of 0.36 inches per minute was used to stressthe specimens to failure. The flexural tests were performed at ambienttemperature and humidity conditions according to ASTM D6272, StandardTest Method for Flexural Properties of Unreinforced and ReinforcedPlastics and Electrical Insulating Materials by Four-Point Bending(astm.org/Standards/D6272). Based on the test results, the AC-1291 resinsystem was determined to provide acceptable flexural strength andmodulus as shown by the data in Table 3.

TABLE 3 Flexural Strength And Moduli Results For Plain Weave AS4 CarbonFiber Laminates Prepared With AC-1291 Matrix Resin Sample 1 Sample 2Sample 3 L (support span, in.) 6.1 6.1 6.1 b (sample width, in.) 0.5110.502 0.54 d (sample thickness, in.) 0.188 0.188 0.188 Modulus msi 9.49.4 9.2 Modulus GPa 65 65 63 Max Stress, ksi 68 67 67 Max Stress, Mpa468 464 460

Lap-Shear Strength—The level of adhesion of the anaerobic matrix resinto cured composite laminates was determined. One inch wide strips werecut from a previously cured carbon fiber reinforced (CFR) compositewhich was prepared with AC-1291 resin and 10 plies of AS4 carbon fiberfabric. The fabric was treated with a methylene chloride solutioncontaining 0.2 percent copper acetylacetonate and 1 percent gammaaminopropyltrimethoxysilane. The carbon fiber was wet-out using AC-1291anaerobic resin. A vacuum bag assembly was used to remove oxygen fromthe bagged laminate to promote cure. Vacuum was maintained for one hourat ambient temperature. Cured specimens having dimensions of 6 in.×1in.×0.10 in. were cut from the resulting laminate panel.

The cured one-inch wide composite strips were abraded with 120 gritsandpaper followed by cleaning by wiping with methyl ethyl ketone. Thecleaned strips were then treated with a methylene chloride primersolution containing 0.2 percent copper acetylacetonate and 1 percentgamma aminopropyltrimethoxysilane. After allowing the primer to dry forone hour, AC-1291 anaerobic resin was applied to a one square inch areaof each cured laminate strip adherend. A fiberglass scrim cloth wasapplied to maintain the bondline thickness to 0.010 inches. Two of thestrips were then adhered to each other over the one square inch area.The anaerobic cure was allowed to proceed for twenty four hours. Aftercure, the samples were tested for lap-shear adhesion on an Instron testmachine.

The lap-shear tests were performed according to ASTM D5868 Standard TestMethod for Lap Shear Adhesion for Fiber Reinforced Plastic (FRP)(astm.org/Standards/D5868). The results for the tests, which wereperformed at 73° F., with a crosshead speed of 0.05 inches per minute,are described in Table 4. The average lap-shear strength observed was1,451 psi.

TABLE 4 Lap-Shear Strength Test Results; Carbon Fiber/Epoxy SubstratesBonded with AC-1291 Anaerobic Matrix Resin Lap Shear Strength Specimen(psi) 1 1,302 2 1,518 3 1,456 4 1,473 5 1,506 AVG 1,451 STD 87 CV % 6

Dynamic mechanical analysis testing was performed to determine themechanical properties of cured laminates over a temperature range.Specimens were prepared with anaerobic resin (AC-1291) and treated AS4plain weave carbon fiber fabric. A composite panel was prepared withAC-1291 resin and 10 plies of AS4 carbon fiber fabric. The fabric wastreated with a methylene chloride solution containing 0.2 percent copperacetylacetonate and 1 percent gamma aminopropyltrimethoxysilane. Thecarbon fiber was wet-out using AC-1291 anaerobic resin. A vacuum bagassembly was used to remove oxygen from the bagged laminate to promotecure. Vacuum was maintained for one hour at ambient temperature. Curedspecimens having dimensions of 1.2 in.×0.4 in.×0.1 in. were cut from theresulting laminate panel.

Cured specimens were oscillated at a rate of 1 Hz in the singlecantilever DMA mode while ramping the temperature at a rate of 10°C./min. (5.5° F./min) FIG. 8 shows the DMA results obtained, or theglass transition temperature for the AC-1291 resin system reinforcedwith AS4 carbon fiber (2% promoter on fiber). For comparison, a similarcomposite was prepared and tested using EA-9390 conventional epoxymatrix resin as shown in FIG. 9. FIG. 9 shows dynamic mechanicalanalyses results for EA-9390 resin reinforced with AS4 carbon fiber; Tgafter cure at 250° F. (121° C.). The glass transition temperatureobserved for the anaerobic was comparable to that of the compositefabricated with a standard HENKEL (Bay Point, Calif.) HYSOL EA-9390resin, a two-component epoxy adhesive designed for composite repair.Both test laminates exhibited Tg's of approximately 360° F. (182° C.).

Thermogravimetric analysis (TGA) was performed using a TA InstrumentsModel 2950 Analyzer. Samples of the composites prepared as described in[0062]. However, the specimens for TGA tests were cut to dimensions of0.1 in.×0.1 in.×0.1 in. Specimens were placed in a tared platinum panand heated at a rate of 10° C. per minute (5.5° F. per minute) to 600°C. (1,112° F.). The tests were performed in a nitrogen atmosphere. FIG.10 shows thermal stability of AC-1291 Resin and standard EA-9390 epoxyresin composites; 20 ply construction with AS4 plain weave carbon fiberfabric. The weight loss plots in FIG. 10 compare the thermal stabilityof the AC-1291 resin and the standard EA-9390 epoxy resin composites.The onset of thermal decomposition was approximately 800° F. for boththe developmental resin and the standard resin. The weight percent fiberreinforcement remaining after elimination of the resin fractions isindicated on the plot as well. These weight percents translate intofiber volume percentages in the range of 48-55%.

Void content testing of the composites was determined according to ASTMD2734, Standard Test Methods for Void Content of Reinforced Plastics(astm.org/Standards/D2734). The measured density of composite specimenswas determined by the dry/wet weight method ASTM D792 Method A, StandardTest Methods for Density and Specific Gravity (Relative Density) ofPlastics by Displacement (astm.org/Standards/D792). Specimens preparedaccording to the procedure described in [0054] for preparation of ILSSspecimens. Specimens measuring 1.0 in×0.17 in×0.17 in. were weighed inair then weighed when immersed in distilled water at 23° C. Density wasthen calculated. After the actual density of the composite wasdetermined, the resin fraction was removed using an isothermalthermogravimetric technique. The resin content was then calculated as aweight percent from TGA. By comparing the actual density to thetheoretical density, void content was then calculated. Table 5 lists theresin, fiber, and void volume percents for AC-1291 and EA-9390 laminatesprepared with AS4 plain weave carbon fiber. The percent void content was0.35% for the AC-1291 laminate and 5.43% for the EA-9390 laminate. Thisdifference in void content between the laminates can be seen visually inthe photomicrographs in FIG. 11 and FIG. 12. FIG. 11 showsphotomicrographs of AC-1291/AS4 20 ply composite cross sections. FIG. 12shows photomicrographs of EA-9390/AS4 20 ply composite cross sections.The composite prepared with EA-9390 had a significantly higher level ofporosity compared to the AC-1291 laminate.

What is claimed is:
 1. An anaerobically curing resin system, comprising:acrylate based resin materials; and curing agents, wherein the curingagents comprise peroxide initiators, aromatic amine accelerators, andbenzoic sulfimide, and wherein the curing agents initiate curing inanaerobic conditions at ambient temperature.
 2. The anaerobically curingresin system of claim 1, wherein the acrylate based resin materialscomprise epoxy acrylate, methyl methacrylate, methacrylic acid, tris(2-hydroxy ethyl) isocyanurate triacrylate, isobornyl methacrylate,hexafunctional aromatic urethane acrylate oligomer, tetrahydrofurfurylmethacrylate, and combinations thereof.
 3. The anaerobically curingresin system of claim 1, wherein the peroxide initiators comprise cumenehydroperoxide, t-butylhydroperoxide, p-menthane hydroperoxide,diisopropylbenzene hydroperoxide, pinene hydroperoxide, methyl ethylketone peroxide, and combinations thereof.
 4. The anaerobically curingresin system of claim 1, wherein the aromatic amine acceleratorscomprise N,N-diethyl-p-toluidine, N,N-dimethyl-o-toluidine, acetylphenylhydrazine, N,N-dimethylaniline, N,N-dimethyl-p-toluidine,N,N-dimethyl-p-anisidine, N,N-diethylaniline,N,N-bis-(2-hydroxyethyl)-p-toluidine, and combinations thereof.
 5. Theanaerobically curing resin system of claim 1, further comprising one ormore additives or stabilizers.
 6. The anaerobically curing resin systemof claim 1, wherein the acrylate based resin materials consist of methylmethacrylate, methacrylic acid, epoxy acrylate, tris (2-hydroxyethyl)isocyanurate triacrylate, isobornyl methacrylate, and hexafunctionalaromatic urethane acrylate, and wherein the curing agents consist ofbenzoic sulfimide, N,N-dimethylaniline, and cumene hydroperoxide.
 7. Ananaerobically curing composite matrix resin system, comprising: areinforcement matrix, wherein the reinforcement matrix comprises aresidual coating of organometallic compounds formed by pre-treatment ofthe reinforcement matrix with an activator sizing agent; and ananaerobically curing resin system, wherein the anaerobically curingresin system comprises acrylate based resin materials and curing agents,wherein the curing agents comprise peroxide initiators, aromatic amineaccelerators, and benzoic sulfimide, and wherein the curing agentsinitiate curing in anaerobic conditions at ambient temperature.
 8. Theanaerobically curing composite matrix resin system of claim 7, whereinthe reinforcement matrix is carbon fiber, fiberglass, or quartz.
 9. Theanaerobically curing composite matrix resin system of claim 7, whereinthe activator sizing agent is copper acetylacetonate andgamma-aminopropyltrimethoxy silane dissolved in methylene chloride. 10.The anaerobically curing composite matrix resin system of claim 7,wherein the acrylate based resin materials comprise epoxy acrylate,methyl methacrylate, methacrylic acid, tris (2-hydroxy ethyl)isocyanurate triacrylate, isobornyl methacrylate, hexafunctionalaromatic urethane acrylate oligomer, tetrahydrofurfuryl methacrylate,and combinations thereof.
 11. The anaerobically curing composite matrixresin system of claim 7, wherein the peroxide initiators comprise cumenehydroperoxide, t-butylhydroperoxide, p-menthane hydroperoxide,diisopropylbenzene hydroperoxide, pinene hydroperoxide, methyl ethylketone peroxide, and combinations thereof.
 12. The anaerobically curingcomposite matrix resin system of claim 7, wherein the aromatic amineaccelerators comprise N,N-diethyl-p-toluidine, N,N-dimethyl-o-toluidine,acetyl phenylhydrazine, N,N-dimethylaniline, N,N-dimethyl-p-toluidine,N,N-dimethyl-p-anisidine, N,N-diethylaniline,N,N-bis-(2-hydroxyethyl)-p-toluidine, and combinations thereof.
 13. Theanaerobically curing composite matrix resin system of claim 7, furthercomprising one or more additives or stabilizers.
 14. The anaerobicallycuring composite matrix resin system of claim 7, wherein thereinforcement matrix is carbon fiber, wherein the acrylate based resinmaterials consist of methyl methacrylate, methacrylic acid, epoxyacrylate, tris (2-hydroxyethyl) isocyanurate triacrylate, isobornylmethacrylate, and hexafunctional aromatic urethane acrylate, and whereinthe curing agents consist of benzoic sulfimide, N,N-dimethylaniline, andcumene hydroperoxide.
 15. A method for preparing an anaerobically curedcomposite matrix resin, comprising: pre-treating a reinforcement matrixwith an activator sizing agent by applying a solution of the activatorsizing agent to the reinforcement matrix and allowing the reinforcementmatrix to dry, whereby the reinforcement matrix is coated with aresidual coating of organometallic compounds, to produce a pre-treatedreinforcement matrix; impregnating the pre-treated reinforcement matrixwith an anaerobically curing resin system, wherein the anaerobicallycuring resin system comprises acrylate based resin materials and curingagents, and wherein the curing agents comprise peroxide initiators,aromatic amine accelerators, and benzoic sulfimide, to form a pre-curedcomposite matrix resin; and exposing the pre-cured composite matrixresin to anaerobic conditions, to form the anaerobically cured compositematrix resin.
 16. The method of claim 15, wherein the reinforcementmatrix is carbon fiber, fiberglass, or quartz.
 17. The method of claim15, wherein the activator sizing agent is copper acetylacetonate andgamma-aminopropyltrimethoxy silane dissolved in methylene chloride. 18.The method of claim 15, wherein the acrylate based resin materialscomprise epoxy acrylate, methyl methacrylate, methacrylic acid, tris(2-hydroxy ethyl) isocyanurate triacrylate, isobornyl methacrylate,hexafunctional aromatic urethane acrylate oligomer, tetrahydrofurfurylmethacrylate, and combinations thereof.
 19. The method of claim 15,wherein the peroxide initiators comprise cumene hydroperoxide,t-butylhydroperoxide, p-menthane hydroperoxide, diisopropylbenzenehydroperoxide, pinene hydroperoxide, methyl ethyl ketone peroxide, andcombinations thereof.
 20. The method of claim 15, wherein the aromaticamine accelerators comprise N,N-diethyl-p-toluidine,N,N-dimethyl-o-toluidine, acetyl phenylhydrazine, N,N-dimethylaniline,N,N-dimethyl-p-toluidine, N,N-dimethyl-p-anisidine, N,N-diethylaniline,N,N-bis-(2-hydroxyethyl)-p-toluidine, and combinations thereof.
 21. Themethod of claim 15, wherein the anaerobically curing resin systemfurther comprises one or more additives or stabilizers.
 22. The methodof claim 15, wherein the activator sizing agent is copperacetylacetonate and gamma-aminopropyltrimethoxy silane dissolved inmethylene chloride, wherein the reinforcement matrix is carbon fiber,wherein the acrylate based resin materials consist of methylmethacrylate, methacrylic acid, epoxy acrylate, tris (2-hydroxyethyl)isocyanurate triacrylate, isobornyl methacrylate, and hexafunctionalaromatic urethane acrylate, and wherein the curing agents consist ofbenzoic sulfimide, N,N-dimethylaniline, and cumene hydroperoxide.
 23. Amethod for repairing a composite material having a damaged region,comprising: pre-treating a reinforcement matrix with an activator sizingagent by applying a solution of the activator sizing agent to thereinforcement matrix and allowing the reinforcement matrix to dry,whereby the reinforcement matrix is coated with a residual coating oforganometallic compounds, to produce a pre-treated reinforcement matrix;sizing the pre-treated reinforcement matrix to an appropriate size forapplication to the damaged region of the composite material; placing thepre-treated reinforcement matrix on the damaged region of the compositematerial; impregnating the pre-treated reinforcement matrix with ananaerobic ally curing resin system, wherein the anaerobically curingresin system comprises acrylate based resin materials and curing agents,and wherein the curing agents comprise peroxide initiators, aromaticamine accelerators, and benzoic sulfimide, to form a pre-cured compositematrix resin at the damaged region of the composite material; andexposing the pre-cured composite matrix resin to anaerobic conditions,to form an anaerobically cured composite matrix resin to repair thedamaged region of the composite material.
 24. The method of claim 23,wherein the reinforcement matrix is carbon fiber, fiberglass, or quartz.25. The method of claim 23, wherein the activator sizing agent is copperacetylacetonate and gamma-aminopropyltrimethoxy silane dissolved inmethylene chloride.
 26. The method of claim 23, wherein the acrylatebased resin materials comprise epoxy acrylate, methyl methacrylate,methacrylic acid, tris (2-hydroxy ethyl) isocyanurate triacrylate,isobornyl methacrylate, hexafunctional aromatic urethane acrylateoligomer, tetrahydrofurfuryl methacrylate, and combinations thereof. 27.The method of claim 23, wherein the peroxide initiators comprise cumenehydroperoxide, t-butylhydroperoxide, p-menthane hydroperoxide,diisopropylbenzene hydroperoxide, pinene hydroperoxide, methyl ethylketone peroxide, and combinations thereof.
 28. The method of claim 23,wherein the aromatic amine accelerators compriseN,N-diethyl-p-toluidine, N,N-dimethyl-o-toluidine, acetylphenylhydrazine, N,N-dimethylaniline, N,N-dimethyl-p-toluidine,N,N-dimethyl-p-anisidine, N,N-diethylaniline,N,N-bis-(2-hydroxyethyl)-p-toluidine, and combinations thereof.
 29. Themethod of claim 23, wherein the anaerobically curing resin systemfurther comprises one or more additives or stabilizers.
 30. The methodof claim 23, wherein the activator sizing agent is copperacetylacetonate and gamma-aminopropyltrimethoxy silane dissolved inmethylene chloride, wherein the reinforcement matrix is carbon fiber,wherein the acrylate based resin materials consist of methylmethacrylate, methacrylic acid, epoxy acrylate, tris (2-hydroxyethyl)isocyanurate triacrylate, isobornyl methacrylate, and hexafunctionalaromatic urethane acrylate, and wherein the curing agents consist ofbenzoic sulfimide, N,N-dimethylaniline, and cumene hydroperoxide.